Glycolipids are known to be involved in the formation of ordered functional domains in biological membranes. Since the structural characterization of such domains is difficult, most studies have so far dealt with lipid mixtures containing only one glycolipid component at a time, although biological membranes usually contain several glycolipid species, which can result in more complex structures and phase behavior. Here, we combine classical isotherm measurements with surface-sensitive grazing-incidence X-ray diffraction to investigate the phase behavior and miscibility in Langmuir monolayers of binary glycolipid mixtures. We find that the phase behavior has a subtle dependence on the saccharide headgroup chemistry. For compatible chemistries, molecular superlattice structures formed by one of the glycolipid species are conserved and can host foreign glycolipids up to a defined stoichiometry. In contrast, for sterically incompatible saccharide chemistries, the superlattice is lost even if both species are able to form such structures in their pure forms. Our results suggest that related phenomena may play important roles also in biological contexts.